This invention generally relates to audio transducers. More particularly, the invention relates to improvements in the design of a transducer diaphragm having a pair of elongate resilient webs whose intermediate portions form an expanse that extends generally in a plane and that is mounted for movement in the direction of the plane.
Various types of audio transducers, as exemplified by audio loudspeakers, are known in the prior art. One common form of transducer comprises a cone with an attached electromagnetic motor driving element. The cone is mounted to a frame by a flexible expanse which bounds the perimeter of the cone. This type of transducer is generally characterized by a relatively high diaphragm and coil mass which creates high inertial forces in the diaphragm. These forces limit the ability of the diaphragm to vibrate at high frequencies and thus reduce its frequency response drastically at frequencies above 5 kHz. Conversely, if the diaphragm and coil are of relatively low mass to raise the upper end of the frequency response, the diaphragm has a reduced low frequency response. In addition to a limited frequency response, the cone-shaped diaphragm is typically molded from a paper product which renders it susceptible to changes in relative humidity. This alters the frequency response and limits the life of the transducer.
Another type of loudspeaker known in the art comprises a horn-type speaker having a flat diaphragm which oscillates normal to the plane of the diaphragm in response to activation by an electromagnetic driving element. As with the cone-shaped diaphragm, the flat diaphragm portion is mounted to a frame by means of an annular portion bounding a flat central portion. In some instances, the diaphragm may be suspended from a voice coil to which it is directly attached. With this type of speaker a large horn is required to direct and focus properly the sound waves produced. Again, by reason of the mass of the diaphragm and voice coil, the frequency response of the transducer tends to drop off at high frequencies. Moreover, the transducers just described tend to be very expensive.
Such prior audio transducers generally have a limited bandwidth and are optimized for specific frequency ranges such as low, mid, and high frequencies. To provide adequate frequency response over the entire audio spectrum, three or four types or sizes of transducers must be incorporated into a single cabinet. The additional transducers increase the cost of high quality sound reproduction. Moreover, the use of multiple transducers requires the incorporation of complex crossover networks to isolate audio signals traveling to or emanating from the individual transducers.
U.S. Pat. No. 4,584,439, which is incorporated herein by reference, discloses an audio transducer that overcomes the shortcomings and difficulties indicated above. The embodiment described therein includes a diaphragm having a pair of elongate resilient webs whose intermediate portions form an expanse extending generally in a plane and having curved end portions which extend laterally away from the plane to terminate at remote frame locations. The webs thus appear from a top view as a pair of back-to-back "C"s joined at their midpoints. The expanse is supported in the frame by string-like supports to allow the expanse to move in the direction of the plane. To complete the diaphragm, a wire coil is attached to the expanse and magnets are mounted on opposite sides of the expanse to provide a magnetic field across the expanse. Current in the coil proportional to received audio impulses creates a magnetic field that interacts with the existing magnetic field to vibrate the webs and generate sound waves thereby.
The illustrated embodiment disclosed therein, however, has some physical limitations. The bandwidth is somewhat limited. The lower cutoff frequency is typically around 1200 Hz rather than the hoped-for cutoff of 100 Hz. The diaphragm also incurs reflections of waves in the web material at the locations where the webs terminate in the frame. The reflected waves to some degree distort the amplitude response of the diaphragm by canceling some waves in the web and doubling others so that the amplitude of the sound produced is uneven. The embodiment also exhibits broad band material resonance. The shape of the frame, combined with the diaphragm and string-like materials, produces distorting resonance around 1 kHz. Still another problem with the prior design is the limited horizontal dispersion. Sound from the transducer radiates forward in about a 30.degree. arc from the central expanse, leaving much of a room without direct exposure to the sound.